1. Field of the Invention
This invention relates generally to airbag firing systems and, more particularly, to an optimization of a single-point frontal airbag fire threshold for determining when to fire an airbag.
2. Discussion of Related Art
Airbags are a common safety device in today's motor vehicles. Airbags work, in conjunction with seatbelts, to provide additional restraints for passengers during collisions. Upon impact in a crash, one or more airbags will deploy to provide a cushion for the occupants of the vehicle. The deployed airbag will prevent an occupant from being thrown forward during a crash and from impacting the front console, steering wheel or windshield. Side airbags are also becoming more common. These airbags deploy from the interior sides of an automobile and protect the occupants against lateral injuries.
Once an airbag is deployed, it is a costly and time-consuming process to service the airbag for reuse. This process may involve replacing one or more parts of the airbag system at considerable time and expense to the vehicle's owner. Therefore, it is not desirable to deploy an airbag during every collision. For instance, during a low speed crash the impact of the vehicle may not be sufficient to injure its occupants, and deploying the airbag would not serve a substantial protective function. Drivers often back into inanimate objects or frequently bump into other cars at stoplights or in parking lots. For these types of collisions, airbag deployment is not desirable. Rugged driving conditions or rapid braking may frequently appear to the airbag sensors to be a collision, but in these conditions deploying the airbag is not desirable.
Additionally, it is known that the airbag deployment itself may cause injuries. The rapid force of the airbag deployment causes the physical injuries. During high velocity collisions, the benefit of the protection afforded by the airbag far outweighs any injury its deployment may cause; however, at low speeds the airbag deployment itself may cause physical injuries while serving a substantially diminished protective function. Due to this tradeoff, it is desirable to deploy airbags only for high speed crashes, front impact crashes and other accidents that have a high rate of injury.
Since airbags are not utilized in all types of crashes, it is necessary to determine when a crash occurs and to differentiate between the severity of different collisions. Vehicles typically have one sensor mounted in the passenger compartment of the vehicle, although it is possible to use more than one sensor. The sensor may be of any known type, such as a switch, an accelerometer, or a velocity, force or position sensor. The sensor sends its information to the airbag control system, which uses the readings to determine when to properly fire the airbag. The airbag control system must differentiate between sensor readings that are caused by hard braking from high velocities (a non-fire condition) and a head-on crash at a low velocity (a fire condition), for instance, and fire the airbag appropriately.
Crash severities are measured against a barrier equivalent speed. For instance, a hundred mile an hour change in velocity may occur over a long period of time. This change may be due to hard braking, and airbag deployment would not be necessary. This would equate to a small barrier equivalent velocity one that would be less than the firing criterion. A fifteen mile an hour crash may occur into a rigid structure. For this crash airbag deployment may be necessary, even though the total change in speed is less than for the one hundred mile an hour braking change. The barrier equivalent velocity for the fifteen mile an hour crash would be higher than the firing condition, and the airbag would fire.
Several problems make an absolute determination of airbag firing difficult. Different collision velocities and different collision angles will produce varying sensor readings. From these changing readings, the collision velocity and type must be accurately determined. It is not physically possible to test all crash conditions when building the airbag deployment system, nor is it economically feasible. Therefore, since the sensor response for all collision conditions is not known, it is extremely difficult to determine a firing algorithm that will only fire during a collision and will not fire during any other time.
The performance and reliability of an airbag during crash conditions can be quantified based on three quantities: fire-time, threshold and fire-rate. The fire-time is the point in time when the airbag module is triggered. Threshold, also called "threshold velocity", is a velocity above which the airbag should fire, and below which the airbag should not fire. Fire-rate is the percentage of times, probability, that the airbag will fire for a given velocity. Ideally, an airbag should always deploy for a collision above the threshold velocity and it should never deploy for a collision below the threshold velocity.
The optimization of both the sensor readings and the three performance quantities is a difficult endeavor. For any given crash that involves an airbag-equipped vehicle, the severity of the impact must be assessed. This information is then used to determine if the airbag should be deployed. The accuracy of determining whether to deploy the airbag can be greatly improved if it is made after the crashed has ended, because the greatest amount of crash information will have been obtained. However, the effectiveness of the airbag will have reached a minimum; the earlier an airbag is fired during a crash, the greater its effectiveness. If the airbag is deployed when the vehicle crash is initiated, then its effectiveness will be maximized. However, if an airbag is instantly deployed, little or no information on the crash severity will be obtained and either the airbag will deploy in all crashes regardless of velocity, or the fire-rate curve will be very broad. As a result of these considerations, there is an inherent tradeoff between fire-rate and time-to-fire in an airbag deployment system.
Present methods for determining the firing threshold in an airbag deployment system use substantially subjective methods. First, a number of crash tests are run on a vehicle. These crash tests are varied over a number of speeds and a number of levels of severity. In some of the crashes it will be desirable to fire the airbag; however, in some tests it would not be desirable to fire the airbag. The sensor readings from all these tests are recorded. Next, the requirements for firing the airbag are determined. These requirements include primarily setting two quantities: fire-velocity and fire-time. In a crash above the fire-velocity, the airbag should always fire, and in a crash below the fire-velocity the airbag should never fire. Also, if the airbag is going to fire, then it should fire before the fire-time. After the requirements are determined, the sample data is manually fit to make a firing threshold curve. Various parameters in the curve may be adjusted to fit the data, so that in each sample crash the appropriate firing determination is made.
Airbag fire conditions are complex, and there exists a tradeoff between firing early, for maximum airbag effectiveness, and firing late, for maximum accuracy. In light of these conflicting requirements, it is often not possible to fit the data taken from the crash tests into a continuous firing threshold curve. Currently, when the data and the requirements conflict, the problem is solved by altering the requirements so that a new and achievable threshold curve can be established. This process is done subjectively by the designers of the system. There is currently no way to quantify the tradeoffs made between early fire time and accuracy in altering the requirements to fit a threshold curve. Therefore, optimization of the tradeoffs is not possible in the current process for establishing the threshold firing curve. The current methods only marginally accomplish the required task of determining the threshold curve and making the compromise between the tradeoffs, because the current methods are most often subjective.
Airbag systems are becoming increasingly complex. New systems integrate more than one sensor into the vehicle. Additionally, multi-stage airbag systems are being constructed. These systems fire one stage of the airbag at a determined time and may also fire a second stage at a later time. The firing criteria for the different stages may be based on different velocity or acceleration characteristics. The current methods of manually fitting a firing threshold to the known data points will not be adequate to determine a firing threshold for these new and more complex systems.
In light of these considerations, there exists a need for an improved method of determining airbag fire conditions. This method must take into account the varying sensor readings for different types and speeds of crashes. The method must balance the tradeoff between achieving a rapid fire-time and the length of time needed to acquire accurate data about a crash's severity, and it must be adaptable to emerging airbag technologies.